We speak three languages in our family --- Marathi, which is our native language, besides Hindi and English, which are both common throughout India. Our older kid grew up speaking only Marathi at home. She picked up Hindi with here playmates in the apartment, and learnt English at day-care. She is good at all three now. Our second child is going through the same process now. There was no serious confusion in either child's mind, besides the occasional mistake of crossing words from one language to another.
The older kid said her first sentence consisting of three words at 18 months, but the younger kid did not. By that stage, both had a large vocabulary of words to express themselves, and even larger vocabulary of words that they understood.
EDIT: Missed the detail about "since birth". So no, my example does not answer your question, since they were exposed to only one language --- Marathi --- until 18 months.
The older kid said her first sentence consisting of three words at 18 months, but the younger kid did not. By that stage, both had a large vocabulary of words to express themselves, and even larger vocabulary of words that they understood.
EDIT: Missed the detail about "since birth". So no, my example does not answer your question, since they were exposed to only one language --- Marathi --- until 18 months.